Ineffective thermal communication between a heat source and a thermal management system like a heat sink can hamper dissipation of excess heat from a system. Electronic components, such as high-power LEDs and high-power circuitry, for example, are continually decreasing in size and becoming ever more powerful, thereby generating loads of excess heat that are increasingly being concentrated in smaller and smaller spaces. Growing production of excess heat and concentration thereof can make effective heat removal especially problematic.
Failure to remove excess heat from an electronic system can result in significant consequences such as, for example, overheating, reduced conduction, higher power requirements than normal, and/or the need for clock-down operation to avoid board burnout and device failure. In many instances, operational modifications are employed to limit the production of excess heat, rather than altering the system architecture to promote better heat removal. Thus, certain systems may be operated in an inefficient condition compared to how they would otherwise be operated if removal of the excess heat did not prove so problematic.
Ineffective heat conduction can be especially prevalent in circuit boards of various types, particularly printed circuit boards (PCBs). PCBs and similar circuit boards are thermal insulators by the very nature of their construction. Specifically, PCBs generally employ thermally insulating substrates (e.g., glass fiber epoxy composites like FR4, which has a thermal conductivity value of 0.25 W/m·K), upon which appropriate electronic circuitry and various board components are disposed. The low thermal conductivity values of PCB substrates can make removal of excess heat from electronic systems rather difficult. Very little excess heat is capable of being removed via the leads due to their typically small size. In addition, conventional lead solder is not especially thermally conductive (e.g., about 1/10th or less than that of more thermally conductive metals, such as copper). Similarly, the metal traces (circuitry) defined in PCBs are typically thin and embedded in the substrate, thereby not allowing much heat dissipation to take place.
In some cases, heat dissipation may be improved by adding a thermal ground plane to one face of the PCB. However, this design prevents the use of that particular face of the board for adding components to increase the complexity and functionality of the PCB, thereby limiting its design and use. Another option sometimes used is incorporation of a thicker copper layer in the center of the board as a highly conductive thermal path. This approach poses manufacturing challenges to incorporate such a large and thick copper sheet, particularly due to its vastly different thermal expansion properties, thereby leading to mechanical stresses during PCB manufacture and operation. In fact, the mechanical stress resulting from thermal expansion may render the PCB ineffective for the very thermal conduction purpose it was designed for. Again, the complexity of the board may be limited unless the board can be effectively connected to a thermal plane.
Current architectures for removing excess heat from PCBs or similar circuit boards only offer one way out for the excess heat, through a heat sink or similar thermal management device in thermal communication with a component generating the excess heat, such that the excess heat is conveyed away without passing through the PCB substrate. This approach allows heat dissipation to take place from only a single face of the PCB. Growing heat removal demands have often necessitated the use of increasingly large and heavy heat sinks, such as machined copper blocks, to maximize the surface area in thermal communication with the source of excess heat to promote better heat dissipation. Even so, it can still be difficult to dissipate excess heat due to the poor thermal conductivity of the PCB substrate and limited available space for mounting the heat sink. Moreover, copper and similar metal blocks are heavy, which may be undesirable for payload-sensitive operations, and they may be subject to disengagement during rough transport or other conditions of use, thereby precluding effective conveyance of excess heat. Thus, present approaches for affecting removal of excess heat from PCBs and similar circuit board assemblies are becoming increasingly less effective, thereby hampering further advances in board technology as a whole.
It is possible, in principle, to introduce features into PCBs or similar circuit board assemblies to improve heat conduction, including to the opposite face of the board relative to where the excess heat is being generated, but this approach can lead to problems in its own right using conventional metal processing techniques. For example, one or more holes (vias) may be drilled through a PCB substrate directly beneath a component producing excess heat, and the holes may be loaded with a highly conductive material, such as copper. This approach could increase the overall thermal conductivity value of the PCB. However, direct liquid casting of metals into vias is not compatible with the board materials that are presently in use (metal processing temperatures >1000° C. in comparison to much lower polymer melting points for materials typically used as substrates). As such, vias are often packed with rosin or a similar filler and then galvanically capped at the ends or left open with just a thick metal plating (e.g., copper) on the via walls (i.e., the via barrel). Coating thicknesses upon the via walls are typically in the range of ˜25 microns. Although metal plating approaches may be effective to promote electronic conductivity between components in various board layers, metal plating only marginally increases the thermal conductivity profile, since the surface area of the metal exposed at the board face remains rather small.
As such, thermal vias of the foregoing type are usually limited to small diameters, oftentimes a diameter of 1 mm or less, because electroless copper plating does not allow for effective filling of large holes or high aspect ratio vias. Since it would take a very long time (days) to completely fill vias having a diameter in this size range, a large number of incompletely filled vias are often used for promoting effective heat removal. However, this approach may be insufficient for removing large quantities of heat (e.g., >100 W/cm2). Moreover, an excessive amount of vias can promote mechanical weakness in a PCB.